Christopher starts counting at 31, and he counts by threes. If 31 is the 1st number that Christopher counts. what is the 11th number that he counts?
Explanation: What is the first number that he counts? $31$ What is the second number that he counts? $\begin{align*}&31 + 3 \\ &= 34\end{align*}$ What is the third number that he counts? $\begin{align*}&31 + 3 + 3 \\ &= 31 + (2 \times 3) \\ &= 37\end{align*}$ What is the 11th number that he counts? $\begin{align*}&31 + (10\times3) \\ &= 31 + 30 \\ &= 61\end{align*}$